GALVANISM. replaced by iron, which is nearly as electro-negative in which a needle should turn when influenced as either in concentrated nitric acid. The electromotive forces of the various cells expressed in volts, are Grove's, 1-92; Bunsen's, 1.88; Daniell's, 1079; Smee's, 0.47; and Wollaston's, 0-39. The resistance in Daniell's cell is much greater than in Grove's or Bunsen's. Different forms of the Zinc-carbon Battery.-Marie Davy Cell: This cell is used extensively on French telegraphic lines with the best results. It is much smaller in size than the smallest Daniell cell, the containing glass vessel being little more than 3 inches in height. It has been found that 38 elements do as much as 60 couples of Daniell, and keep in action twice as long, requiring no replenishing for half a year. The chemical action is much the same as that of Daniell's. The cell is thus charged: powdered mercurous sulphate, HgSO4, is treated with water, a basic insoluble salt is formed, and falls to the bottom of the vessel, but a small amount of a mercurous salt is left dissolved in the water. The clear liquid is decanted off and a paste is left. The carbon is placed in the porous cell, and the vacant space is filled with the paste thus got The glass vessel outside containing the zinc is charged with pure water and a little of the decanted liquid. The connections are made by straps of lead instead of copper, to avoid the action of the mercurous salt on the latter metal. The small quantity of mercurous salt in the zinc cell has the excellent effect of keeping the zinc constantly amalgamated. -Bunsen's Bichromate Cell: Bunsen sometimes charges his cells with a liquid consisting of from 100 to 150 parts, by weight, of water, to which 12 parts of bichromate of potash and 25 parts of sulphuric acid have been added. No porous cell is needed with this charge, the carbon being kept from touching the zinc by hempen cords or the like. The Bichromate Cell, as generally used in this country, is thus made up. The liquid with the carbon in the porous cell is in the proportion of 10 oz. of water to 1 oz. of bichromate of potash and 5 oz. of sulphuric acid. In Bunsen's bichromate charge, zincic sulphate is formed and potassium chrome alum. In the last-named bichromate cell, zincic chloride is formed in the zinc cell, and potassic and sodic chrome alum in the porous cell. GALVANOMETERS.-The two most reliable evidences of the strength of the galvanic current are, its power to deflect the magnetic needle, and to effect chemical decomposition. To measure one or other of these, is the object of a galvanometer or voltameter. A magnetic galvanometer shews the strength of the current by the amount of the deflection of the needle, and shews its direction by the way in which it deflects. The manner by a current is easily kept in mind by Ampere's rule: Suppose the diminutive figure of a man to be placed in the circuit, so that the current shall enter by his feet, and leave by his head; when he looks with his face to the needle, its north pole always turns to his left. The deflecting wire is supposed always to lie in the magnetic meridian. The Astatic Galvanometer, or Galvanometer, is used either simply as a galvanoscope, to discover the existence of a current, or as a measurer of the strengths of weak currents. When a needle is placed under a straight wire, through which a current passes, it deflects to a certain extent, and when the wire is bent, so as also to pass below the needle, it deflects still more. This is easily understood from the above rule. The supposed figure has to look down to the needle when in the upper wire, and to look up to it in the lower wire, so that his left hand is turned in different ways in the two positions. The current in the upper and the lower wire moves in opposite directions, thus changing in the same way as the figure; and the deflection caused by both wires is in the same direction. By thus doubling the wire, we double the deflecting force. If the wire, instead of making only one such circuit round the needle, were to make two, the force would be again doubled, and if several, the force (leaving out of account the weakening of the current caused by the additional wire) would be increased in proportion. If the circuits of the wire be so multiplied as to form a coil, this force would be enormously increased. Two needles, as nearly the same as possible, placed parallel to each other, with their poles in opposite ways, as shewn in fig. 17, and suspended, so as to move freely, by a thread without twist, have little tendency to place themselves in the magnetic meridian, for the one would move in a contrary direction to the other. If they were exactly of the same power, they would remain indifferently in any position. They cannot, however, be so accurately paired as this, so that they always take up a fixed position, arising from the one being somewhat stronger than the other. This position is sometimes in the magnetic meridian, sometimes not, according as the needles are less or more perfectly matched. Such a compound needle is called astatic, as it stands apart from the directing magnetic influence of the earth. If an astatic needle be placed in a coil, as in fig. 17, so that the lower needle be within the coil, and the upper one above it, its deflections will be more considerable than a simple needle, for two reasons: in the first place, the power which keeps the needle in its fixed position is small, and the needle is consequently more easily influenced; in the second place, the force of the coil is exerted in the same direction on two needles instead of one, for the upper needle being much nearer the upper part of the coil than the lower, is deflected alone by it, and the deflection is in the same direction as that of the lower needle. An astatic needle so placed in a coil constitutes an astatic galvanometer. One of these instruments is shown in fig. 18. Round an ivory bobbin, AB, a coil of fine copper wire, carefully insulated with silk, is wound, its ends being connected with the binding screws, 8, 8. The astatic needle is placed in the bobbin, which is provided n S Fig. 17. GALVANISM. with a vertical slit, to admit the lower needle, and a lateral slit, to allow of its oscillations, and is suspended by a cocoon thread to a hook supported Fig. 18. in size. The glass before and behind is so close to the mirror as to allow it no more play than the small range the instrument requires, and the mirror, when at rest, just clears the sides of the box, and nothing more. In other galvanometers, the mass of the movable part, and its comparatively weak magnetism, make the indications of the needle tediously slow. In this instrument, the moving mass is small, and its magnetism comparatively great, and this, combined with the viscosity of the air in the narrow chamber and the powerful directive force of the external magnet, gives an immensely greater rapidity of indication, with even increased sensitiveness. Tangent Galvanometer.-This instrument is shewn in fig. 19. It consists essentially of a thick strip of copper, bent into the form of a circle, from one assumed that the needle in any direction it lies holds the same relative position to the disturbby a brass frame. The upper needle moves on This being the case, it ing power of the ring. a graduated circle; the compound needle hangs is easy to prove that the freely, without touching the bobbin. The whole is strengths of currents cirincluded in a glass case, and rests on a stand, sup-culating in the ring are ported by three levelling screws. When used, the proportionate to the tanbobbin is turned round by the screw, Q, until the gents of the angles of needle stands at the zero point, and the wires through deviation of the needle. which the current is sent are fixed to the binding Thus, if the deflection screws. The number of degrees that the needle caused by one galvanic deflects may then be read off. couple was 45°, and of another 60°, the relative strengths of the currents sent by each would be as the tangent of 45° to the tangent of 60°-viz., as 1 to 173. The needle can never be deflected 90°, for as the tangent of 90° is infinitely large, the strength of the deviating current must be infinitely great, a strength manifestly unattainable. The tangent galvanometer can consequently be used to measure the strongest The sensitive galvanometers, now fast taking the place of all others, are those designed by Sir William Thomson. In fig. 18A, which represents the dead-beat S R Fig. 18A. currents. Fig. 19. Voltameter. This was invented by Faraday for testing the strength of a current. Fig. 20 shews how it may be constructed. Two platinum plates, each about half a square inch in size, are placed in a bottle containing water acidulated with sulphuric acid; the plates are soldered to wires which pass galvanometer, the general action of these is shewn. C is a bobbin filled with fine covered wire set on a stand. In the tube A, which forms the centre of the bobbin, a tubular box fits, in the middle of which hangs a circular mirror made of microscopic glass, slightly concave, suspended by a fine silk fibre. To the back of this mirror are stuck four fine magnetic needles. The mirror and needles weigh of a gramme (grain). The fibre by which the mirror hangs is too feeble to give anything like a quick set to it. To accomplish this, a powerful magnet is laid outside the instrument, and placed so that the mirror lies in the way required for observation. About a metre in front of the mirror, a graduated scale, 88, is placed, and immediately below it a lamp L. A pencil of light, I, is sent from L, and reflected (R) back to the scale, where it comes to a focus at the zero point. A slight deflection of the needles, caused by a current in the coil, makes a decided displace- up through the cork of the bottle; binding screws ment in the spot of light on the scale. The com- are attached to the upper ends of these wires; a partment in which the mirror is hung is very limited glass tube fixed into the cork serves to discharge Fig. 20. Fig. 21. GALVANISM. the gas formed within. When the binding screws connection with the binding screw, S. The key, are connected with the poles of a battery, the water H, fits the projecting staple of either cylinder, in the bottle begins to be decomposed, and hydrogen and can consequently turn both. As the brass and oxygen rise to the surface. If, now, the outer cylinder, C', is turned in the same direction as the end of the discharging tube be placed in a trough hands of a watch, it uncoils the wire from the of mercury (mercury does not dissolve the gases), wooden cylinder, C, making it thereby revolve and a graduated tube (fig. 21), likewise filled with in the same way. When the wooden cylinder is mercury, be placed over it, the combined gases rise turned contrary to the hands of a watch, the reverse into the tube, and the quantity of gas given off in takes place. The number of revolutions is shewn a given time measures the strength of the current. by a scale placed between the two, and the fraction The voltameter chooses as a test the work which of a revolution is shewn by a pointer moving the current can actually perform, and establishes a on the graduated circle, P. When the binding uniform standard of comparison. The indications screws, S and S', are included within a circuit, say of the tangent galvanometer are comparable only S with the positive, and S' with the negative pole, with its own, but the quantity of gas discharged the current passes along the wire, on the wooden by the voltameter, corrected for pressure and tem- cylinder, C, till it comes to the point where the wire perature, is something quite absolute. However, crosses to the brass cylinder, C; it then passes up by comparing the indications of both instruments the cylinder, C', to the spring and binding screw, S. with each other when placed in the same circuit, The resistance it encounters within the rheostat is an absolute standard may likewise be got for the met only in wire, for as soon as it reaches the large tangent galvanometer. If, for instance, the current cylinder, C', the resistance it encounters up to S' may given by a battery should give 60 cubic centimetres be considered as nothing. When the rheostat is to of gas in a minute, and produced at the same be used, the whole of the wire is wound on the time a deflection of 45° in the galvanometer, the wooden cylinder, C, the binding screws are put into ratio of 60 to the tangent of 45°-viz., 60 to 1 = the circuit of a constant cell or battery along with 60, is constant, for correct measurements of the a galvanometer, astatic or tangent. If, now, the strength of currents, however taken, must bear to resistances of two wires are to be tested, the each other a constant ratio. If the angle of devia- galvanometer is read before the first is put in the tion for another current was 30°, we have therefore circuit. After it is introduced, in consequence of only to multiply 60 by the tangent of 30°, to ascer- the increased resistance offered by it, the needle tain the amount of gas that would be liberated by falls back, and then as much of the rheostat wire is a current of that strength in a minute. This found, unwound as will bring the needle back to its former we know the meaning of a deflection of 30° of the place. The quantity of wire thus uncoiled in the galvanometer in question in a perfectly comparable rheostat is shewn by the scales, and is manifestly standard. The plates of the voltameter must be equal in resisting power to the introduced wire. small, for when they are large, a small quantity of The first is then removed, the rheostat readjusted, electricity is found to pass without decomposing the and the second wire included, and the same unwater. It is found also that a minute quantity of winding goes on as before. To fix our ideas, let the oxygen forms hydric peroxide with the water, the quantity of wire unwound in the first case be and remains in solution, so that when very great 40 inches, and in the second case 60 inches; 40 accuracy is required, the hydrogen alone ought to inches of the rheostat wire offer as much resistance to the current as the first wire, and 60 inches of it as much as the second. We have thus 40 to 60 as the ratio of the resistances of the two wires. The wire of the rheostat, from its limited length, can only be comparable with small resistances; and where great resistances are to be measured, supplementary resistance coils of wires, of a known number of ohms, are introduced into the circuit, or removed from it, as occasion requires, leaving to the rheostat to give, as it were, only the frac tional readings. This being premised, it will be easily understood how the following results have been ascertained. It is proved, for instance, that the resistances of wires of the same material, and of uniform thickness, are in the direct ratio of their lengths, and in the inverse ratio of the squares of their diameters. Thus a wire of a certain length offers twice the resistance of its half, thrice of its third, and so forth. Again, wires of the same metal, whose diameters stand in the ratio of 1, 2, 3, &c., offer resistances which stand to each other as 1, 4, t, &c.; therefore, the longer the wire the greater the resistance; the thicker the wire the less the resistance. The same holds true of liquids, but not with the same exactness. For this reason, the larger the plates of a galvanic pair, and the nearer they are placed to each other, the less will be the resistance offered to the current by the intervening liquid. The following table of the resistances, expressed in ohms, offered by a wire one metre long and one millimetre in diameter at 0° Centigrade, has been determined by Dr Mathiessen: Silver annealed, 0.01937; copper annealed, 002057; gold annealed, 0-02650; aluminum annealed, 0-03751; zinc, 0.07244; platinum annealed, 0-1166; iron annealed, 0.1251; be measured. RESISTANCES TO THE CURRENT.-It is found that the dimensions and material of substances included in the circuit exercise an important influence on the strength of the current. It is of the greatest importance to ascertain the relative amount of the resistance offered by conductors of various forms and materials. The rheostat, invented by Wheatstone, is generally employed for this purpose, and for this object is constructed so as to introduce or withdraw a considerable amount of highly resisting wire from the circuit without stopping the current. It is shewn in fig. 22. Two cylinders, C', C, about 6 inches in length, and 1 inch in diameter, are placed parallel to each other, both being movable round their axis. One of them, C', is of brass, the other, C, is of well-dried wood. The wooden cylinder has a spiral groove cut into it, making forty turns to the inch, in which is placed a fine metallic wire. One end of the wire is fixed to a brass ring, which is seen in the figure at the further end of the wooden cylinder; and its other end is attached to the nearer end (not seen in the figure) of the brass cylinder, C. The brass ring just mentioned is connected with the binding screw, S, by a strong metal spring. The further end of the cylinder C, has a similar Fig. 22. GALVANISM. tin, 0-1701; lead, 0·2526; mercury, 1-2247; German equations with two unknown quantities, from which silver, 0-2695. With copper at 32° F. as 1, the e and I can be easily found. In doing so, we must following liquids stand thus: Saturated solution adopt a unit of resistance. The unit proposed and of the sulphate of copper, at 48° F., 16,885,520; determined by the British Association, the B.A. ditto of chloride of sodium at 56° F., 2,903,538; unit, or the ohm, is the only one now used in this sulphate of zinc, 15,861,267; sulphuric acid, diluted country. The resistance of the liquid of the pair to, at 68° F., 1,032,020; nitric acid, at 55° F., would be expressed in units of this, and the electro976,000; distilled water, at 59° F., 6,754,208,000. motive force in absolute units or centimetres of gas, The slightest admixture of a foreign metal alters with a circuit offering a unit of resistance. the resistance very decidedly: per cent of iron in THE EFFECTS OF THE GALVANIC CURRENT may be copper wire increases the resistance more than 25 classified under physiological, mechanical, magnetic, per cent. It has been found also that the resist-heating, luminous, and chemical. The mechanical ance offered by a wire increases as its temperature effects relate to the mutual attraction or repulsion rises. It is almost needless to add, that the of one current to another, or to a part of itself. conducting powers of metals are inversely as their These, along with the magnetic effects, will be found specific resistances, the least resisting being the treated of under MAGNETO ELECTRICITY. The heatbest conducting. ing and luminous effects have been partly discussed under ELECTRIC LIGHT. We shall here only further refer to the heating of wires, and to the galvanic spark. The luminous effects of galvanic electricity of very high tension will be given under INDUCTION COIL. The chemical effects have been already referred to, but a fuller consideration of these will now be given under the head Electrolysis in this article. = e The physiological effects, as shewn by the convulsions of Galvani's frog preparation, were the first observed manifestation of the current. Frog-limbs, as prepared by Galvani, when included in a circuit, form a galvanoscope of excessive sensibility, which rivals the finest galvanometer in delicacy of indicaThe application. There is one peculiarity in their action which The limbs contract only deserves to be noted. Ohm's Law. This law is singularly in accordance with experimental results. It assumes that the electro-motive force for a particular galvanic pair is constant, and that the strength of the current it produces is the quotient which results from dividing it by the resistance of the circuit. This resistance arises from two sources, the first being the resistance within the cell offered by the exciting liquid, and the second the interpolar resistance. If e represent the electromotive force; l, the resistance within the cell; w, the interpolar resistance; and S, the strength of the current, or the quantity of electricity actually transmitted, the statement of the law for one couple stands thus: S tion of the law in a few particular cases will best illustrate its meaning. If we increase the number of cells to n, we increase the electromotive force n times, and at the same time we increase the liquid resistance n times, for the current has n times as much of it to travel, then S = If u be nl + w small compared with nl—that is, if the external connection be made by a short thick wire-it may be neglected, and so S = This shews that one nl 7 cell gives in these circumstances as powerful a current as a large battery. But if nl be small with respect to was in the interpolar circuit of an electric telegraph battery-nl may be neglected, and S = ne = ne e = ne when the circuit is completed and broken, and remain undisturbed so long as the current passes steadily through them. The more frequently, therefore, the current is stopped and renewed, the greater is the physiological effect. The same is experienced when a current is passed through the human body. When the terminal wires of a battery are lifted one by each hand, except it consist of a very large number of cells, almost the only sensation felt is a slight shock on completing and breaking the circuit. Du Bois Reymond, the great authority on animal electricity, states that the nerves of motion are affected only by changes in the electric tension of the current, whereas the nerves of sensation are affected not only by these, but also by the steady continuance of the current, and that the excitation of the nerves dependent on the changes of tension Frictional electricity in this way owes its superior increases with their frequency and suddenness. physiological power to the instantaneous nature of its discharge. It is only currents of great tension which affect the ordinary human nerves. The giving a brilliant electric light, for instance, may poles of a battery of 50 Bunsen cells, capable of be handled without much inconvenience. This may be attributed partly to the non-conducting nature of the skin. If the current enter the body by a cut or wound, the sensation is affected even when the current is weak. The physiological effect is also much heightened by moistening the hands with salt and water, or by holding metal handles instead of If the exterior resistance is small, wires, so as to improve the conducting connection. Here we learn that the strength of the current increases directly as the number of cells. We may learn from the same that the introduction of the coil of long thin wire of a galvanometer into such a circuit, introducing but a comparatively small increase of resistance, causes a very slight diminution of the current strength. If, again, we increase the size of the plates of a galvanic pair n times, the section of the liquid is proportionately increased, so that whilst the electromotive force remains the same, the cell resistance diminishes n times; therefore S = S = ne nl may be neglected, and S: and the strength is thus shewn to increase n times. These are only a very few of the conclusions arrived at by this law. With the aid of a tangent galvanometer, which gives the value of S expressed in absolute magnetic units, or centimetres of voltameter gas, we ascertain e and for any pair. By making two observations with two wires of known resistance separately included in the circuit, we have two simple Another cause of this insensibility may be attributed to the fact that the current is not restricted, as it is in part of the frog preparation, to the nerve, but passes through all the conductors of the system. The nerves of the palate can be affected by a very feeble current; that of sight by one proceeding from a battery of one or two cells, and that of hearing by a battery of some 30 cells. See ELECTRICITY, MEDICAL Heating Effects.-When a strong current passes through thin wires, an intense heat is produced, GALVANISM. sufficient to bring them to a white heat, and to &c., which, being electro-negative (anions), always fuse them. This is turned to practical use in appear at the positive pole. Moreover, the propor exploding gunpowder, in engineering and mining tions of the volumes of the two gases being that of operations. Two wires of a battery placed at a safe their chemical combining volumes, reminds us that, distance are insulated from each other, and their when a body is decomposed, its components are ends, which are connected by a fine iron wire, are always separated in the proportions in which they sealed up in a tin cartridge filled with gunpowder, were united, viz., those of their chemical equivalents. and laid in the exploding charge. When all is If the tubes of this apparatus were graduated, it adjusted, the battery connection is completed, and would serve for a voltameter. If, instead of one the current making the iron wire red hot, ignites the such voltameter included in the circuit, we had gunpowder in the cartridge, and that again the several, we should find that, whatever amount of charge. In this way, all danger is avoided. Experi- gas was liberated in one of these, the same amount ments on the heating effects of the current through would be liberated in all, and that independent of wires have proved that the heat developed is propor- the size of the plates, and amount of acid in tional to the resistance of the wires, and to the squares each. We learn, therefore, that the chemical power of the strength of the currents; and that the strength of the current is the same at every point of of the current being the same, any length of wire may the circuit where it is manifested. If, instead of be heated to the same redness. two or three voltameters in the circuit, we had Electrolysis is that branch of the science of gal- one and two decomposing cells of the following vanism which treats of the laws and conditions of description. A test tube, having a platinum wire, electro-chemical decomposition. As this decomposi- on which the glass has been fused, passing through tion is generally attended by electro-chemical com- the bottom, is partially filled with stannous bination, it is sometimes difficult to distinguish chloride, which is kept fused by the heat of a spiritelectrolysis from the inore general subject of Electro- lamp. The platinum wire at the bottom of the chemistry, which embraces all chemical changes tube forms one electrode, and one descending from resulting in or from the galvanic current. In one the top forms the other, dipping below the fused case, however, the application of the term is strictly chloride. If, then, this cell be included in the correct-viz., where decompositions are effected by circuit along with the voltameter, and a similar electrodes (poles, see ANODE), which are not attacked cell containing fused plumbic chloride, so that the by the elements of the electrolyte (the substance current enters the tubes by the upper electrodes, decomposed) discharged at them. Throughout the and leaves by the lower, the water, stannous chloride article, there have been frequent allusions to electro- of tin, and plumbic chloride, are decomposed simulchemical changes, but taneously by the current passing through each. In here we shall discuss the voltameter, hydrogen and oxygen are disengaged; more particularly the in the tubes, metallic tin is deposited at the lower laws of electro-chemical electrode of the one, and lead at the other; whilst decomposition. No sub- chlorine is liberated at the upper electrodes of both. stance is decomposed If, now, the quantity of hydrogen, tin, and lead thus by the current so long set free be weighed, the weights found correspond as it is in a solid or with their chemical formula: H,O, SnCl2, PbCl, gaseous state, and it must first be brought H=2, Sn=118, Pb-207. From such experiments as these, we conclude that electrolytes are resolved to a liquid state, either under the action of the current into anions and cations by solution or fusion, which appear at their respective electrodes in the proportion of their atomic weight, or multiples of on it. The decompo- their atomic weights. It is not only in cells exterior sition of water by to the battery that this law holds, but in the cells platinum plates 18 always taken as the of the battery itself. If the battery which effected the above decomposition consisted of six cells, for the equivalent atoms of hydrogen, tin, and lead separated without the battery, equivalent atoms of zinc in each cell would have been dissolved, and an equivalent disengagement of atoms of hydrogen at each of the copper plates, if the cells were one-fluid. The above law holds not only for compounds whose elements enter into combination with their usual atomicity, but for those in which the elements, though the same, change their atomic equivalents. Thus, if the same current pass through two decomposing cells, one containing a solution of the cuprous chloride (CuCl), and the other of the cupric chloride (CuCl,), the same quantity of chlorine will be disengaged in both, but twice as much copper is deposited in the first as in the second. copper alone changes its atomicity, hence the change in the amount of it in the consecutive cells. The accuracy of the electrolytic law is somewhat compromised by the fact that liquids possess, to a certain extent, the power of conducting, physically, electricity without electrolytic action, so that all that passes in this way is chemically lost. Fortunately, the error thus introduced is very small, and can be therefore practically disregarded. Fig. 23. before the current acts type of electrolytic action. Fig. 23 represents a very convenient apparatus for the purpose. A glass basin is made so as to admit a cork below, through which two wires pass having slips of platinum plate soldered to them above. Two glass tubes, open below, are hung over the plates, to hooks projecting from an upright support. The bowl is filled with acidulated water; and the tubes, after being filled with the same, are inverted, and hung with their lower ends enclosing the plates. When the wires projecting downwards from the cork are connected with the poles of the battery, hydrogen rises from the negative, and oxygen from the positive electrode, to fill each its separate tube. As the decomposition proceeds, twice as much hydrogen is liberated as oxygen. When the tubes are filled, they may be removed and examined. The oxygen thus obtained smells strongly of ozone. Hydrogen is here the type of the metals or other electro-positive substances (cations), which, during electrolysis, are always disengaged at the negative electrode; and oxygen of the salt radicals, chlorine, iodine, sulphur, Here the ELECTRO-METALLURGY is the art of depositing, electro-chemically, a coating of metal on a surface 605 |